1. Field of the Invention
This invention relates to atomic time standards utilizing a diode laser to optically pump vapor in a gas cell which is also excited by a microwave signal regulated with respect to the atomic transition frequency of the vapor and from which a time standard signal is generated. More particularly, it is related to such a time standard in which the wavelength of light generated by the diode laser is electronically stabilized against variations in temperature.
2. Background of Information
Atomic frequency standards of the gas cell type provide very accurate timing information. They operate on the principal that certain element vapors have very precise atomic transition frequencies. The vapor is excited by a microwave signal tuned with respect to the atomic transition frequency. Light of another specific atomic transition in a beam passed through the gas cell is absorbed. A photodetector which detects light which has passed through the vapor is used in a feedback circuit to lock the microwave signal to the microwave atomic transition frequency of the vapor. A precise timing signal is output by the microwave circuitry with a stable frequency which is a fraction of the atomic transition frequency to which the microwave circuit it tuned. Commonly, the microwave circuit modulates the microwave signal applied to the gas cell between two frequencies on either side of the atomic transition frequency with the output signal being generated from an average of the two microwave frequency signals.
The operation of the above-described gas cell type atomic standard is dependent upon a stable light source. For this invention, the light source is a laser which is controlled with a feedback signal from the photodetector. Current efforts have been directed toward miniaturizing the gas cell type atomic standard. An example of such a cell-type atomic frequency standard is disclosed in U.S. Pat. No. 5,192,921. Such a miniaturized atomic frequency standard utilizes a diode laser as the light source and utilizes cesium vapor in the gas cell. A key feature of the operation of this atomic frequency standard is the laser diode excitation of a narrow ((&lt;1 GHz)) hyperfine optical resonance transition at .apprxeq.852 nm.
Simple internal cavity diodes can be locked to the required transition through a combination of temperature and current tuning. Temperature tuning is required for two purposes even though tuning can be achieved by varying the current to the diode. Current tuning can only shift the wavelength a relatively small amount compared with the variation of the diode laser center frequency as manufactured, even for diodes selected for a given wavelength region. However, even if diode lasers were manufactured with extremely close tolerances in maximum gain wavelength, current tuning does not vary the wavelength smoothly. Instead, it exhibits a phenomenon called "mode hopping" in which the wavelength shifts abruptly with current, then smoothly for a small wavelength region, and then abruptly and so on. That is, the shift is more like a staircase than a ramp.
In order to have the smooth ramp region at the desired wavelength, temperature regulation is required. The desired temperature control is not stringent (.apprxeq..+-.1.degree. C.), but quite specific. Therefore, if the clock must operate in a wide thermal ambient, considerable power must be expended in heating or cooling the laser. In addition to compensating for variations in ambient temperature, cooling power is required to remove the electrical power dissipated in the laser if the desired operating temperature is near or below the ambient temperature. In diode lasers today which typically required 40.apprxeq.100 mW electrical power and generate .apprxeq.10 mW optical power, the bulk of the cooling power is required to remove the electrical power dissipated. On the other hand, it is anticipated that diode lasers for the miniature atomic frequency standards will require &lt;10 uW optical power which can be obtained with &lt;10 mW electrical power. Under these conditions, or by establishing a relatively high operating temperature, the bulk of the thermal control power will be due to fluctuations in the ambient temperature. Control of the temperature of the laser adds to the complexity, the cost and also the size of the atomic frequency standard, the latter of which is an important consideration in development of miniature atomic frequency standards.
There is a need, therefore, for an improved atomic frequency standard utilizing a diode laser light source which does not require stringent thermal control.
There is a related need for such an improved atomic frequency standard which is simple, economical, and compact.